Yeasts are known for a high nutritional value, and have been used as feed supplements, both for animals, and to some extent for humans. Yeasts have a high protein content in general, and probably contain more B-group vitamins than most other protein sources, B.sub.12 a notable exception. Pichia pastoris strains are becoming known for usefulness as high-productivity strains in single cell protein (SCP) processes, but none have been reported with high tryptophan content.
TABLE I __________________________________________________________________________ ESSENTIAL AMINO ACIDS OF SOME PROTEIN SOURCES.sup.(a)(b) Pichia Candida pastoris Amino Egg Spirulina Saccharomyces lipolytica Pseudomonas Alcaligenes Penicillium NRRL Y- Acids Wheat White maxima cerevisiae (gas oil) (methanol) eutrophus notatum 11430.sup.(c)(d) __________________________________________________________________________ Lysine 2.8 6.5 4.6 7.7 7.8 5.3 8.6 3.9 -- Threonine 2.9 5.1 4.6 4.8 5.4 4.5 4.5 -- -- Methionine 1.5 3.2 1.4 1.7 1.6 1.8 2.7 1 -- Cystine 2.5 2.4 0.4 -- 0.9 0.3 -- -- -- Tryptophan 1.1 1.6 1.4 1.0 1.3 -- 1.1 1.25 1.1 Isoleucine 3.3 6.7 6.0 4.6 5.3 3.9 4.6 3.2 -- Leucine 6.7 8.9 8.0 7.0 7.8 7.0 8.5 5.5 -- Valine 4.4 7.3 6.5 5.3 5.8 5.9 7.1 3.9 -- Phenylalanine 4.5 5.8 5.0 4.1 4.8 4.2 4.0 2.8 -- __________________________________________________________________________ .sup.(a) Data from p. 110 Industrial Applications of Microbiology, J. Riviere (Wiley 1977), except for P.p. NRRL Y11430. .sup.(b) The values shown are g amino acid/100 g protein. .sup.(c) Tryptophan amino acid analysis of NRRL Y11430 made by colorimetric methods. .sup.(d) Tryptophan content in Pichia pastoris Y11430 is 1.1 g/100 g protein (as shown), which equals 0.67 g/100 g weight of cells, grown on glucose substrate.
TABLE II ______________________________________ Essential Amino Acid composition of Various Food Yeasts.sup.(a)(b) S. S. cerevisiae Candida utilis S. fragilis cerevisiae Amino Acid molasses sulphite liquor milk whey beer ______________________________________ Lysine 8.2 6.7 8.8 7.3 Valine 5.5 6.3 6.6 5.2 Leucine 7.9 7.0 9.9 6.3 Isoleucine 5.5 5.3 5.5 5.7 Threonine 4.8 5.5 5.5 4.8 Methionine 2.5 1.2 1.5 1.2 Phenylalanine 4.5 4.3 3.9 4.4 Tryptophan 1.2 1.2 1.5 1.1 Histidine 4.0 1.9 2.5 1.5 Arginine 5.0 5.4 4.9 4.7 ______________________________________ .sup.(a) Data from p. 110 Industrial Applications of Microbiology, J. Riviere, (Wiley 1977), except for P.p. NRRL Y11430. .sup.(b) The values shown are g amino acid/100 g protein.
As can be seen above, tryptophan content generally is relatively low in yeasts. Needed are Pichia pastoris yeasts that have good productivity in single cell protein production, but with much increased (enhanced) tryptophan content.